Research On Sequence, Sedimentology And Favorable Reservoir Of Carboniferous-permian In Huanghua Depression

Taking Carboniferous-Permian in Huanghua Depression as the research object,the lithology and tectonic evolution characteristics of its provenance were analyzed,the sequence framework was established,and the types,distribution and evolution of sedimentary facies were determined together with the types,characteristics and controlling factors of favorable reservoirs using multidisciplinary theories and methods,which can provide a basis for the hydrocarbon exploration in the study area.Through rare earth elements analysis,heavy mineral analysis,statistical identification of clastic particles and dickinson diagram analysis,together with previous studies,the provenance characteristics of the Carboniferous-Permian in the study area were studied.The Inner Mongolia uplift,which is an active continental margin of andean type in the Carboniferous-Permian,is the provenance of the study area.The detrital material in the basin mainly comes from volcanic arc,back-arc sedimentary cap,plutonic rock and basement metamorphic rock.The provenance region underwent three stages of tectonic evolution during the Late Carboniferous-Middle Permian,including the back-arc overall uplift stage in early Hercynian,the back-arc fold-thrust stage in middle Hercynian and the overall fold-thrust stage in late Hercynian.In the Late Carboniferous and Early Permian（early Hercynian）,back-arc overall uplift was the mainly tectonic activity of the provenance area,and the detrital material mainly came from the back-arc sedimentary cap.In the early-middle Middle Permian（middle Hercynian）,large-scale back-arc fold-thrust occurred in the provenance area,and the plutonic rocks and basement metamorphic rocks in the back arc were exposed to the surface,together with the back-arc sedimentary cap,they provided detrital material to the basin.In the late Middle Permian（late Hercynian）,overall fold-thrust occurred in the provenance area,the volcanic arc also underwent uplifting and destroyed and provided detrital material,along with back-arc sedimentary cap,plutonic rocks,and basement metamorphic rocks,to the basin.Based on the observation of core and thin section,and combined with the data of logging,seismic data,SEM,XRD,whole rock carbonate content,major element,rare earth element and previous studies,the sequence and system tract boundaries of Carboniferous-Permian in the study area were recognized.According to the second-order sequence boundaries,such as the boundaries among early Hercynian,middle Hercynian and late Hercynian basin filling sequences and the boundaries among Carboniferous-Permian,lower Paleozoic and Mesozoic,combined with the third-order sequence boundaries,such as erosional basal surface of incised valley,transition surface of sedimentary system,large area sedimentary intermittent surface,discontinuity surface of paleoclimate,the Carboniferous-Permian in the study area can be divided into three second-order sequences（TS1-TS3）and eight third-order sequences（SQ1-SQ8）.SQ1 corresponds to Benxi Formation,SQ2 and SQ3 correspond to the lower and upper submembers of Taiyuan Formation,respectively,SQ4 corresponds to Shanxi Formation,SQ5 corresponds to lower Shihezi Formation,SQ6 corresponds to Wanshan submember of upper Shihezi Formation,SQ7 corresponds to Kuishan submember and Xiaofuhe submember of upper Shihezi Formation,SQ8 corresponds to Shiqianfeng Formation.TS1 consists of SQ1,SQ2,SQ3 and SQ4,TS2 is composed of SQ5 and SQ6,and TS3 consists of SQ7 and SQ8.SQ1 consist of low stand system tract and transgressive system tract,both SQ3 and SQ3consist of transgressive system tract and high stand system tract,the other third-order sequences are composed of low stand system tract,transgressive system tract and high stand system tract.Through observing core and thin section,meanwhile,analyzing data such as logging,particle size,SEM,whole rock carbonate content,trace element and rare earth element,the sedimentary characteristics of Carboniferous-Permian in the study area were studied.The Carboniferous-Permian in the Huanghua Depression has developed barrier coastal facies,carbonate platform facies,delta facies,meandering river facies,and braided facies.The barrier coastal facies mainly include lagoon subfacies,tidal flat subfacies,the tidal channel subfacies,and barrier island subfacies.Carbonate platform facies mainly consist of open platform subfacies and restricted platform subfacies.The delta facies are mainly composed of delta plain subfacies and delta front subfacies.Meandering river facies mainly consist of riverbed subfacies,bank subfacies and flood plain subfacies.Braided river facies mainly include riverbed subfacies and flood plain subfacies.SQ1,SQ2 and SQ3 mainly develop barrier coastal facies and carbonate platform facies,SQ4 mainly develops delta facies,SQ5,transgressive system tract and high stand system tract of SQ7 and SQ8 develop meandering river facies,SQ6 and the low stand system tract of SQ7 develop braided river facies.In Carboniferous-Permian,the sedimentary facies of the study area experienced a gradual transition from epeiric sea to delta and eventually evolved into river.Through observation of core,outcrop and thin section,and combined with analytical test data,the favorable reservoir characteristics of Carboniferous-Permian in Huanghua Depression were studied.There are three favorable reservoir types in the Carboniferous-Permian of Huanghua Depression,namely tuff,dolomite and sandstone.The lithology of tuff reservoir is vitric tuff,the reservoir space are mainly intercrystalline micropores formed after devitrification and metasomatism.The pores are characterized with small size and large number,which result in the reservoir featured by high porosity and low permeability.According to the reworking degree of hydrothermal fluid on tuff reservoir,the tuff reservoir can be classified into typeⅠand typeⅡ.Dolomite reservoir was formed by the hydrothermal fluid reformation on limestone,the reservoir is composed of matrix and veins.Microcrystalline and fine crystalline dolomite are developed in the matrix,while hydrothermal minerals such as coarse crystalline dolomite and iron dolomite are developed in veins.The reservoir space types are mainly fractures,dissolution pores and intercrystalline pores.Sandstone reservoirs can be divided into weathered crust reservoir,thermal cycle convection modification reservoir and upwelling current modification reservoir.The weathered crust sandstone can be divided into weathered clay layer,vertical seepage zone,seasonal fluctuation zone,horizontal undercurrent zone and deep stagnant zone from top to bottom.The horizontal undercurrent zone is the main reservoir development zone,which is characterized by low gamma and high resistance in logging.The reservoir space types include intergranular pore,mold hole,intergranular pore and intercrystalline pore,and the reservoir is characterized by high porosity and high permeability.The thermal cycle convection modification reservoir is developed in buried hill with large stratigraphic inclination,and favorable reservoir occurs in the middle and upper part of sand body.The reservoir space types include intergranular pores and intercrystalline pores,and the reservoir is characterized by moderate porosity and moderate permeability.The upwelling current modification reservoir occurs in deep buried hill with relatively gentle stratigraphic inclination.Organic acids and CO₂generated by source rocks of Carboniferous-Permian coal series and Cenozoic source rocks in the maturation process rose along the fault and entered into sandstone,dissolving the unstable components in sandstone.The reservoir space types are mainly intergranular pores,mold holes,intragranular pores and intercrystalline pores,followed by fractures,and the reservoir is characterized by moderate porosity and moderate permeability.The development of the Carboniferous-Permian reservoirs in Huanghua Depression is controlled by the provenance,sedimentary environment and tectonic activities.Volcanic materials provided by the provenance are the material basis of tuff reservoir.Volcanic materials and feldspars originating from the provenance provide material basis for the formation of secondary pores in sandstone reservoir.The provenance characteristics control the vertical distribution of sandstone reservoir and dolomite reservoir.Tidal flat and tidal channel environment provide favorable conditions for the preservation of tuff material and promote the development of tuff reservoir.The development of carbonate platform facies provide the material basis for the formation of dolomite reservoir.Tectonic activities provides channels for the rise of deep hydrothermal fluid,promoting the development ofⅠtype tuff reservoir and hydrothermal dolomite reservoir.At the same time,tectonic activities formed different types of buried hills in the basin,which controlled the plane distribution of different types of sandstone reservoirs.Beidagang buried hill is a favorable area for the development of tuff reservoir,and the reservoir is mainly distributed in the high stand system tract of SQ3.Beidagang buried hill and Koucun buried hill are favorable areas for the development of dolomite reservoir,and the reservoir is mainly distributed in the transgressive system tract of SQ1-SQ3.High buried hill such as Beidagang buried hill,Qibei buried hill,Qinan buried hill,Yangsanmu buried hill and Koucun buried hill are favorable areas for the development of weathered crust sandstone reservoir and thermal cycle convection modification reservoir,and the reservoirs are mainly distributed in TS2 and TS3.Middle and low buried hills such as Chenghai buried hill,Kongdian buried hill and Wangguantun buried hill are favorable areas for the development of upwelling current modification reservoir,and the reservoir is mainly distributed in TS2 and TS3.